The Times found that vaccine exemptions for kindergartners -- which allow them to enroll without having state-mandated immunizations -- had more than doubled in the last decade, leaving hundreds of schools in the state vulnerable to outbreaks.

The rise in unvaccinated children appears to be driven by affluent parents choosing not to immunize. Many do so because they fear the shots could trigger autism, a concern widely discredited in medical research.

The report sparked intense reactions both from those who believe vaccines are harmful and from others upset that a rising number of parents are opting out of immunizations. Check out your local schools using The Times schools database and use our interactive map to see which Southern California school might be at risk.

"I don't understand why an 'outbreak' is being 'triggered' if the vast majority of students (and others) HAVE been vaccinated."

"Can you please explain what risk the kids without the shots are to the kids that have had all their shots?"

"If people choose to immunize their own children, what is it that they are worried about?"

"What do medical professionals say about advising pregnant women not to eat food high in mercury and then giving babies vaccines that contain mercury?"

What are the autism rates among immunized versus non-immunized children?

The risk of autism was the same for children immunized and not immunized for measles, mumps and rubella, according to a study of more than 500,000 Danish children published in the New England Journal of Medicine in 2002. Researchers analyzed data on all children born in Denmark between 1991 and 1998 using the Danish Psychiatric Central Register, a database that has information on every diagnosis given in a Danish psychiatric hospital and outpatient clinic. Overall, there was no increase in the risk of autistic disorder or other autistic-spectrum disorders among vaccinated children as compared with unvaccinated children, researchers concluded.

I don't understand why an "outbreak" is being "triggered" if the vast majority of students (and others) HAVE been vaccinated. Are the vaccines not working, such that vaccinated persons exposed to measles, for example, are still developing disease?

Outbreaks can occur when there are enough non-immunized people in a group, allowing a disease can spread from one person to another. In a measles outbreak last year that infected a dozen children and infants in San Diego, none had been vaccinated. Three of them were infants too young to receive the inoculation.

If a high percentage of a group is vaccinated, even those who are not immunized are effectively protected by what is known as "herd immunity." In practice, this means a child either too young to be fully vaccinated or unable to get vaccinations for medical reasons, such as allergies or a compromised immune system, would still be safe.

But the U.S. Centers for Disease Control and Prevention estimate that infections can spread quickly within a group when as few as 5% to 10% lack vaccination.

Can you please explain what risk the kids without the shots are to the kids that have had all their shots?

Children are largely protected against the diseases from which they are vaccinated. Still, vaccines can fail to work in a small percentage of cases, when the immune system does not respond to the inoculation. In those cases, they may fall ill. When "herd immunity" is intact, a child whose vaccination failed to work would still be protected, as would others who could not be vaccinated for medical reasons.

If people choose to immunize their own children, what is it that they are worried about? If their kids have the shot, then isn't their child immune to that disease? Wouldn't outbreaks not be of concern to them since they are so-called protected? Talk about control freaks!

Although rare, vaccinations can fail to work. And non-immunized children may come in contact with infants too young to be immunized, and those who cannot be be vaccinated for medical reasons, because they either have a weakened immune system or are allergic to a vaccine's ingredients

In the San Diego case, three infants who came in contact with the infected boy at the doctor's office were sickened. In all, 70 children were quarantined there, including children at a daycare center attended by one of the infected babies.

On a larger scale, public health officials say high rates of immunization help eradicate diseases; smallpox was eradicated in 1979 based on an intense immunization campaign. Until a measles vaccine was available in the mid-1960s, the disease killed 450 a year in the U.S. and caused 4,000 cases of encephalitis, according to the CDC. Measles, which had been eradicated in England and Wales, has seen a resurgence there because of dropping vaccination rates. British public health officials have declared the disease once again endemic.

What does the medical community have to say about the fact that during pregnancy women are advised not to eat certain fish, due to mercury content, yet when the babies are born they are injected with some vaccines that contain mercury?

There is a difference between the mercury found in fish and the mercury once found in routine childhood vaccines. The version found in fish is known as methylmercury, which can accumulate in fish and humans. The kind that was used in vaccines is known as ethylmercury, which does not accumulate in the body and is actively excreted via the gut, according to the World Health Organization.

The distinction is not trivial, wrote Dr. Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia, who wrote about the issue in his book, "Autism's False Prophets," published in 2008.

"An analogy can can be made between ethylalcohol, contained in wine and beer, and methylalcohol, contained in wood alcohol," Offit wrote. "Wine and beer can cause headaches and hangovers; wood alcohol causes blindness."

The mercury-based preservative thimerosal has been taken out of routine childhood vaccinations since 2001. The preservative does remain in some flu shots; thimerosol-free flu shots are also available.

The preservative was removed from vaccines as a precautionary measure, even though there was no proof that thimerosal was harmful. Researchers later looked to see if thimerosal was causing autism; subsequent studies have concluded that the mercury-based preservative was not at fault. One such study found that autism rates continued to rise in California even after the preservative was phased out of routine childhood vaccinations.